1. Field of the Invention
The present invention relates to a dielectric ceramic composition which is widely used in high frequency electronic components, more particularly, to a low-temperature cofired dielectric ceramic composition with a high dielectric constant and a low dielectric loss.
2. Description of the Prior Art
For use in a high frequency range (˜2 GHz), chip type components such as LC filters, require that their electrodes be high in electrical conductivity. Ag or Cu is selected for internal electrode due to their high electrical conductivity. Ag and Cu have melting points of 961° C. and 1,083° C., respectively, which are both much lower than those of Ni (1,455° C.) or Ag—Pd.
Dielectric material must have lower sintering temperatures than the melting point of the internal electrode. In the case that Ag or Cu is employed as electrodes, available dielectric materials can be therefore selected from only a narrow range.
Generally, LTCC materials using Ag as an internal electrode are composed mainly of glass frit in combination with ceramic fillers for improving strength and dielectric properties. And its sintering temperature is about 900° C. or lower.
However, such compositions are, for the most part, found to have dielectric constants of 10 or less, which are too low to apply the compositions for LC filters. For use in LC filters, dielectric compositions are required to show a high dielectric constant, a low dielectric loss (high Q value), and a stable temperature coefficient of resonant frequency.
For instance, dielectric ceramic compositions with high dielectric constants allow the reduction of the size of the electrodes, making it possible to miniaturize devices. Additionally, such dielectrics are very useful in reducing insertion loss. Further, stable temperature coefficients of resonant frequency are helpful in stabilizing high-temperature properties of dielectrics.
Development of LTCC materials with high dielectric constants has largely been investigated in two manners: one is to develop new systems that can be sintered at 900° C. or lower; the other is directed to composite systems comprising low-temperature sintering aids or glass frit on the basis of conventional dielectric materials of high dielectric constants.
Usually, the former is Bi-based systems. These systems, however, have difficulty in being used in practice due to reactivity with electrodes, and poor reproducibility.
In association with the latter, there is known a technique in which a CaO—Sm2O3—Nd2O3—Li2O—TiO2 composition (K. H. Yoon et. al., Jpn. J. Appl. Phys., 35[9B] 5145 (1996)) with a sintering temperature of 1,300° C. or higher is combined with the sintering aid B2O3—Li2O to reduce the sintering temperature to 1,100° C. However, 1,100° C. is still too high to conduct the co-firing of Ag electrodes.